1. [Field of the Invention]
The present invention relates to an information recording apparatus, which can switch the image dot density, for example, to an information recording apparatus such as a laser beam printer and, more particularly, to an information recording apparatus and an image recording method, in which bit map data representing a character or a figure is printed after smoothing processing, so that the edge of the printed character or figure can be smoothed to improve printing quality.
2. [Description of the Prior Art]
In recent years, laser beam printers has been widely used as output devices for computers. In particular, compact printers having a resolution of about 300 dpi (dots/inch) have become increasingly popular since they are compact and inexpensive.
As shown in FIG. 77, a laser beam printer comprises a printer engine 1801 for actually forming a printing image on a photosensitive drum on the basis of dot data, and transferring the formed image onto an output sheet, and a printer controller 1802, connected to the printer engine 1801, for receiving code data supplied from an external host computer 1803, generating page information consisting of dot data (bit map data) on the basis of the code data, and sequentially transmitting the dot data to the printer engine 1801. The host computer 1803 loads a program from a floppy disk 1804 containing an application software program, starts the application software program, and serves as, e.g., a wordprocessor.
The printing operation process in the printer controller 1802 will be described below with reference to FIG. 78.
In FIG. 78, reference numeral 1114 denotes an image memory for storing bit map data (image data) for one page; 1115, an address generator for generating an address for the image memory 1114; 1116, an output buffer register for converting image data read out from the image memory 1114 into an image signal VIDEO; 1117, a sync clock generator for generating an image clock signal VCLK synchronized with a known beam detect signal (BD signal) as a horizontal sync signal; 1118, a CPU for controlling the overall controller; 1119, a printer I/F as a signal I/O unit with a printer engine 1201; and 1120, a host I/F serving as a signal I/O unit for communication with an external host apparatus such as a personal computer.
In the above arrangement, an operation when the image signal VIDEO is supplied to the printer engine will be described below.
When image data for one page are prepared in the image memory 1114, a printer controller 1802 (1202) sends a print request signal PRINT to the printer engine 1801 (1201). Upon reception of the signal PRINT, the printer engine 1801 starts the printing operation. When the printer engine is ready to receive a vertical sync signal VSYNC, it sends a signal VSREQ to the printer controller 1802. Upon reception of the signal VSREQ, the printer controller 1802 sends the vertical sync signal VSYNC to the printer engine 1801, and counts a predetermined period of time from the signal VSYNC, so that the printing operation can be performed from a predetermined position in the subscanning direction.
Upon completion of the counting of the predetermined period of time, the address generator 1115 sequentially generates addresses from the start address of image data stored in the image memory 1114, thereby reading out the image data. The read-out image data are input to the output buffer register 1116 for each main scanning line. The output buffer register 1116 counts the image clock signal VCLK after the BD signal is input, so that the printing operation is started from the predetermined position in the main scanning direction. Thereafter, the register 1116 supplies data for the corresponding printing line to the printer engine 1801 as the image signal VIDEO synchronized with the signal VCLK. The printer engine 1801 performs the above-mentioned image forming operation.
The operations described above are performed in units of printing pages, so that the printing operation can be always performed at the same position on paper sheets.
However, in recent years, demand has arisen for an increase in resolution of printing output, not excepting by laser beam printers.
Thus, an increase in resolution of the laser beam printer may be proposed. For example, assume that the resolution is increased to 600 dpi, twice the common 300 dpi. In this case, if the increase in resolution is realized by simply increasing the capacity of the image memory needed for the printer controller, a capacity four times that required for 300 dpi is required, resulting in an expensive printer.
In addition, when the same printing speed as that for 300 dpi is to be obtained, the output frequency of image data must be four times that for 300 dpi, and the printer controller must be operated at a speed four times that for 300 dpi.
Thus, a method of converting printing data of a recording pixel into printing data, in which the dot density is higher than the recording dot density only in the main scanning direction, may be proposed. In this method, recording data of a recording pixel and its surrounding pixels are referred to, and smoothing processing is normally performed.
In the smoothing processing, when the resolution of the printer is switched from, e.g., 300 dpi to 600 dpi, proper smoothing processing cannot be performed.
A laser beam printer using the electrophotography technique is also used in an output apparatus of a computer, an output unit of a facsimile apparatus, a so-called digital copying machine for printing image data read by an image scanner, or the like.
The laser beam printer used in these apparatuses performs a printing operation at a resolution of, e.g., 300 dpi.
In this case, as shown in FIG. 4, a character or a figure is drawn and printed by black dots (.circle-solid. marks) and white dots (O marks), which are printed at positions on a 300-dpi matrix. FIG. 4 shows a dot pattern of English letter "a". At the resolution of 300 dpi, the interval between adjacent dots is about 85.mu.. It is generally known that a person can perceive up to about 20.mu.. As compared to this, the edge portion of a character or figure formed by dots looks nonsmooth at the above resolution (about 85.mu.), and high printing quality cannot be warranted.
In order to solve this problem, the following approaches can be tried.
As the first approach, a method of simply increasing the resolution (e.g., 1,200 dpi) may be adopted. However, in this case, a bit map memory having a capacity 16 times (=4.times.4) that used at 300 dpi is required to express the identical area, resulting in a very expensive apparatus.
As the second approach, the following method is known. That is, a buffer memory having a small capacity is added without increasing the capacity of the bit map memory, and printing data of a pixel of interest is modulated by referring to dot data around the pixel of interest to be printed, thereby equivalently increasing the resolution in one or both of the main scanning and subscanning directions.
The techniques described in U.S. Pat. Nos. 4,437,122 and 4,700,201 are methods of referring to only a pixel of interest and eight pixels around the pixel of interest, in correcting the pixel of interest to be printed. In the method of this type, since a reference region--of the surrounding pixels which are referred to is narrow, it can be recognized that the pixel of interest is a portion of a curve, but the curvature of the curve to which the pixel of interest belongs, cannot be recognized. In particular, an almost horizontal or vertical edge portion cannot be detected. Therefore, since optimal correction cannot be performed according to the curvature, it is difficult to optimize the smoothing effect.
The technique described in U.S. Pat. No. 4,847,641 refers to a wider region than the above-mentioned two techniques, and can recognize the curvature of a curve to which a pixel of interest belongs. In this technique, however, although the reference region is wide, each of the matching patterns to be matched refers to only a portion of the reference region. Therefore, this technique suffers from the following drawbacks.
As the first drawback, it cannot be identified whether or not a pixel of interest is a portion of a binary halftone image such as a dither image or an image formed by an error diffusion method. For this reason, although the smoothing effect can be provided to a character image, some dots constituting a dither image or halftone pixels formed by the error diffusion method may often be erroneously smoothed. For example, FIG. 9-a partially shows a 4.times.4 dither image. In FIG. 9-a, if a limited region around a pixel of interest 5f is referred to, it is erroneously "recognized" that the pixel of interest is a portion of a character or figure. For this reason, the pixel of interest 5f is undesirably converted from a white pixel into a pixel having non-zero density. Therefore, a halftone image suffers from a local change in image density. As a result, possibility of image deterioration (e.g., formation of a pseudo-edge) is high.
As the second drawback, it cannot be identified whether or not a pixel of interest belongs to a portion of an image in which pixels are crowded (concentrated). For example, FIG. 9-b shows an image constituted by a group of crowded dot lines. In this case, pixels for which the density of dots must be changed to smooth lines are those indicated by ".DELTA. marks" or "x marks" in FIG. 9-c. As can be seen from FIG. 9-c, a pixel to be changed is adjacent to or in the vicinity of pixels which are changed for a pixel adjacent to the pixel to be changed. As a result, the resolution of the image is decreased. Pixels are complicatedly crowded not only in crowded line images, but also in a small character or a complicated character such as many kanji. In this case, a pixel of interest, which is changed in the smoothing processing, is adjacent to pixels to be changed for an adjacent image. For this reason, a corresponding pixel (a corresponding line or a side of a corresponding character) cannot be clearly identified from adjacent pixels. As a result, the resolution of an image in the corresponding portion is excessively decreased. As a result, a blurred image is formed, or "moire" noise is generated in an image, resulting in deterioration of image quality. Furthermore, when a pixel is expressed as a halftone pixel within one pixel for the purpose of smoothing in an image crowded portion, density reproduction becomes unstable due to an interaction with adjacent pixels, and is easily influenced by a variation in environment (temperature and humidity). As a result, the smoothing effect varies depending on the environment. For this reason, every time a given character is printed, it may look like another font having a different character pattern.
Of course, the reference region of each matching pattern may be sufficiently widened so as to make it possible to identify whether or not a pixel of interest belongs to a dither image or an image crowded portion. However, the effect of "simplification of a processing circuit" as the object of this technique cannot then be obtained.
As described above, there have been proposed many methods of checking and detecting the kind of feature of a boundary portion, to which a pixel of interest belongs, of an image such as a figure or character.
In contrast to this, as the prior technique about how to change a pixel of interest according to the detected feature of a boundary portion to which the pixel of interest belongs, the following patents are proposed in addition to the above-mentioned prior arts. That is, U.S. Pat. No. 4,933,689, and Japanese Patent Laid-Open Nos. 61-214661 and 61-214666 are known.
However, with the above-mentioned technique, the following problems are posed.
The first problem is associated with to what extent the modulation pixel unit size used when a pixel of interest is changed by a smoothing processing circuit is to be segmented. The modulation pixel size will be described below with reference to FIG. 53-a to 53-d. FIG. 53-a shows an original pixel before smoothing. FIG. 53-b shows a case wherein the modulation pixel size for modulation in smoothing processing is set to be 1/4 the original pixel. FIG. 53-c shows a case wherein the modulation pixel size for modulation in smoothing processing is set to be 1/8 the original pixel. FIG. 53-d shows a case wherein the modulation pixel size for modulation in smoothing processing is set to be 1/16 the original pixel.
As the modulation pixel unit size is made smaller, finer smoothing processing can be performed. However, the driving frequency of a smoothing processing circuit is increased accordingly. Thus, the smoothing processing circuit must be designed using a Bi-CMOS logic or ECL logic, resulting in an expensive circuit.
When the modulation pixel unit size is large, an image is printed while pixels of modulation pixel units are sufficiently resolved, and a portion subjected to the smoothing processing has, not the desired "blurred" appearance but a "whisker"-like appearance. Thus, a desired smoothing effect is not satisfactory.
The modulation pixel unit size which can satisfy both the above-mentioned contradictory conditions, varies considerably depending on the toner particle size. Commercially available laser beam printers normally employ a toner having a toner particle size of 10 to 12.mu. (to be referred to as a normal particle size toner hereinafter).
In recent years, in order to increase resolution, printers which employ a toner having a toner particle size of 5 to 6.mu. (to be referred to as a small particle size toner hereinafter), have been developed or are commercially available. The optimal modulation pixel unit size also varies depending on the laser light-emission response characteristics due to a difference in laser driving circuits, electrophotography process conditions, and the like.
In general, a smoothing processing circuit is often designed as an integrated circuit such as a gate array, such as a CMOS keyboard including 3,000 to 10,000 gates. It is undesirable to develop other gate arrays simply to accommodate differences in toner. On the other hand, when some additional processing functions corresponding to different toners are built in a single gate array, not only is the number of gate keyboards increased, but also a type of logic, with which the integrated circuit can respond to a high frequency, e.g., a Bi-CMOS logic, or ECL logic is required, resulting in an increase in cost. Even for a given toner, the modulation pixel size changes according to a change in laser light-emission rise characteristics or electrophotography process. For this reason, too, it is not practical to prepare different smoothing processing functions corresponding to many conditions in advance.
The second problem is posed when the above-mentioned smoothing processing circuit is applied to a printer engine which can switch the printing dot density. For example, when the printing dot density is switched between 240 dpi and 300 dpi or between 300 dpi and 600 dpi in response to a command, it is difficult to optimize smoothing effects for both printing dot densities. More specifically, even when an algorithm which can improve a smoothing effect for one printing dot density, is used, the smoothing effect for the other printing dot density cannot always be optimized.
The third problem is associated with optimization of smoothing processing for a printer engine which has a function of changing the density of an image to be printed between light and dark levels by, e.g., a method of changing a high voltage to be applied to a developing unit. When the printing dot density is changed, the effect on a portion subjected to smoothing processing is also changed, and as a result, image quality is undesirably deteriorated.
The fourth problem is associated with the influence of use environmental conditions (e.g., temperature, humidity, and the like) of a printer engine on the smoothing effect. When environmental conditions (e.g., temperature, humidity, and the like) are changed, the printing dot density of a printed image is changed, and the effect on a portion subjected to smoothing processing is also changed. As a result, image quality is undesirably deteriorated.